Method for purification of cyclic ester

ABSTRACT

A method of refining a crystal of a crude cyclic ester comprises the steps of supplying the crude cyclic ester to a vertically extending cylindrical refiner ( 1 ) from an inlet ( 3 ) provided at a lower part of the refiner ( 1 ), agitating the crude cyclic ester by an agitator provided in the refiner ( 1 ) to make the crude cyclic ester flow upwardly, refining the crystals of the crude cyclic ester by counterflow-contact between the upflowing crude cyclic ester and a downflowing melted liquid containing a refined crystal component, and taking out refined crystals of the crude cyclic ester from an outlet ( 4 ) provided at an upper part of the refiner ( 1 ).

TECHNICAL FIELD

The present invention relates to a method of refining a cyclic ester.The cyclic ester is a condensation product of a dicarboxylic acid anddiol, or a product made from a hydroxycarboxylic acid dimer which ismade circular by dehydrate-condensation. The cyclic ester is used as astarting material for producing a polyester or polyester amide includinga biodegraded polymer or medical-use polymer.

BACKGROUND ART

It is well known that the cyclic ester is produced by depolymerizinghydroxycarboxylic acid oligomer. There are two types of crude cyclicesters produced by the conventional method. One is a cyclic ester,wherein no solvent is adhered to its crystal, and the other is a cyclicester, wherein a high-boiling point organic solvent is adhered to itscrystal. The crude cyclic esters are refined by the appropriate methodfor each of the crude cyclic esters.

The following references describe the manufacturing method for thefirst-type cyclic ester.

U.S. Pat. No. 2,668,162 describes that a glycolic acid oligomer isreduced to powder and supplied slowly to a reaction vessel, in which itis depolymerized by heating under a super vacuum. Then, the produced gasis cooled for solidification into a crude cyclic ester.

U.S. Pat. No. 4,727,163 describes that a large amount of polyether isblock-copolymerized with a small amount of glycolic acid to make acopolymer. The copolymer is heated under a low pressure to bedepolymerized. Then, the produced gas is cooled for solidification intothe crude cyclic ester.

U.S. Pat. No. 4,835,293 describes that a glycolic acid oligomer isheated into a liquid. The liquid is provided with a flow of nitrogen toincrease the surface area so that the liquid is evaporated from theincreased surface area. The evaporated gas is provided with a flow ofnitrogen and cooled for solidification into a crude cyclic ester.

U.S. Pat. No. 5,326,887 and WO 92/15572A1 describe that a glycolic acidoligomer is heated and depolymerized over a fixed bed catalyst system.Then, produced gas is cooled and solidified into a crude cyclic ester.

The crude cyclic esters produced by the above methods contain impuritiessuch as a hydroxycarboxylic acid oligomer and hydroxycarboxylic aciditself. The crude cyclic esters are purified by re-crystallization usinga solvent such as an isopropanol disclosed by European Patent No.261572, a t-amyl alcohol disclosed by German Patent Publication No.1808939, a carbon tetrachloride disclosed by German Patent PublicationNo. 123473904, an ethyl acetate disclosed by U.S. Pat. No. 4,727,163,and an ether disclosed by Japanese Kokai No. 06-172341. A slurry of thecrystal produced by the re-crystallization is separated into a solid anda liquid by filtering, cleaned by the solvent used in there-crystallization process or an other cleaning liquid, and dried toremove the solvent or cleaning liquid, thus providing a purifiedcrystal.

Japanese Kokai No. 09-328481 describes a method of manufacturing thesecond-type crude cyclic ester, in which a mixture comprising anhydroxycarboxylic acid oligomer and at least one kind of high-boilingpoint organic solvent having a boiling point between 230° C. and 450° C.is heated under normal or reduced pressure at such a temperature as todepolymerize the oligomer. The depolymerized oligomer is dissolved inthe solvent until the remaining rate of the oligomer in the liquid phasereaches 0.5% or less. The heating is further continued at the abovetemperature to depolymerize the remaining oligomer. The produced cyclicester is distilled out with the high-boiling point organic solvent, andthe crude cyclic ester is recovered from the distilled product.

By the method, the cyclic ester is distilled with at least one organicsolvent having a boiling point of 230-450° C., and the crude cyclicester is collected by cooling the distilled product. In this case, anon-solvent may be added in the distilled product, and the solidified orcrystallized cyclic ester is separated into a solid and a liquid. Thethus produced crystal of the cyclic ester includes the high-boilingpoint organic solvent attached thereto, which is difficult to remove bythe usual drying method. However, it is indispensable to remove thehigh-boiling point organic solvent by any method in order to obtain adried crystal.

A conventional method to remove such a liquid as adhered to the crystalof the cyclic ester is to displace and clean the crystal with alow-boiling point cleaning liquid, such as cyclohexane or ether, andthen, remove the low-boiling point cleaning liquid by drying. The dryingprocess is performed at a temperature lower than the melting point ofthe crystal. Since the cyclic ester sublimates, the loss of the crystalis increased if the pressure is reduced excessively during the dryingprocess. In addition, if necessary, the re-crystallization is performedusing an ethyl acetate. Even in this case, the adhered solvent has to beremoved by drying. As fully stated above, in the conventional methodusing the high-boiling point organic solvent, the low-boiling pointcleaning liquid is indispensable to displace the liquid adhered to thecrystal. As a result, the cleaning liquid waste is a mixture comprisingthe high-boiling point organic solvent and the low-boiling pointcleaning liquid.

The method of refining the first-type of crude cyclic ester requiresadditional and complicated processes including the processes for dryingto remove the solvent or cleaning liquid from the crystal surface,cooling to collect the solvent or cleaning liquid removed during thedrying process, evaporating to separate the mixture of the solvent andthe cleaning liquid collected during the cooling process. The dryingprocess is performed at a temperature lower than the melting point ofthe crystal. Also, since the cyclic ester sublimates, the loss of thecrystal is increased under excessively reduced pressure. A solvent, suchas an alcohol, may bring about ester exchange reaction with the cyclicester, and re-crystallization is required a few times to remove theimpurities caught inside the crystal.

The method of refining the second-type of crude cyclic ester alsorequires complicated processes including the processes for drying toremove the cleaning liquid adhered to the crystal surface, collectingthe cleaning liquid removed during the drying process, and disposing torefine and collect a mixture of the high-boiling point organic solventand low-boiling point cleaning liquid. A solvent, such as an alcohol,may bring about ester exchange reaction with the cyclic ester, andre-crystallization is required a few times to remove the impuritiescaught inside the crystal.

Accordingly, it is an object of the present invention to provide amethod of refining the crystal of a crude cyclic ester, which is simpleand applicable to both the first- and second-types of crude cyclicesters.

DISCLOSURE OF THE INVENTION

According to the present invention, crystal are refined by means of avertically extending cylindrical refiner provided with an inlet forsupplying crude cyclic ester at a lower part of the refiner, andprovided with an outlet for taking crystals as a product at an upperpart of the refiner, and provided with an agitator for agitating crudecyclic ester in the refiner

According to the present invention, a method of refining crystals of acrude cyclic ester comprises the steps of supplying the crude cyclicester to a vertically extending cylindrical refiner from an inletprovided at a lower part of the refiner, agitating the crude cyclicester by an agitator provided in the refiner to make the crude cyclicester flow upwardly, refining the crystals of the crude cyclic ester bycounterflow-contact between the upflowing crude cyclic ester and adownflowing melted liquid containing a refined crystal component, andtaking out refined crystals of the crude cyclic ester from an outletprovided at an upper part of the refiner.

According to the invention, the crystal of the crude cyclic ester isrefined by the counterflow-contact between the upflowing crude cyclicester and the downflowing melted liquid containing the refined crystalcomponent in the refiner. That is, inventors have found a method that amother liquid and impurities adhered to the crystal are cleaned off, andthose that are taken inside the crystal are sweat off by thecounterflow-contact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a vertical section of refining equipment according to anembodiment of the present invention.

FIG. 1(B) is an enlarged section taken along line B—B in FIG. 1(A).

FIG. 2 is a vertical section of of refining equipment according toanother embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Cyclic Ester

The cyclic ester is a condensation product comprising a dicarboxylicacid diol, or a product made from a hydroxycarboxylic acid dimer whichis dehydrate-condensed and made circular. An example of thehydroxycarboxylic acid is a glycolic acid, lactaid acid,3-hydroxycarboxylic butylic acid, 4-hydroxycarboxylic butylic acid,3-hydroxycarboxylic valeric acid, or 3-hydroxycarboxylic caproic acid.

In order to provide a finished cyclic ester, it is necessary to refinethe first type of cyclic ester having no solvent or the second type ofcyclic ester having high-boiling point organic solvent adhered thereto.Each of the refining equipment shown in FIGS. 1 and 2 is useful for boththe first and second types of the cyclic esters.

A cylindrical refiner 1 in FIG. 1(A) extends vertically and is providedwith an agitator 2 therein. The agitator 2 comprises, as shown in FIG.1(B), two rotatable shafts 2A extending vertically and having aplurality of agitating fins 2B. The two rotatable shafts 2A may rotatein the same direction or in opposite directions to each other. In eithercase, it is preferable that the agitating fins 2B have inclination withrespect to the horizontal plane so that they create an upflow of thecrude cyclic ester inside the refiner 1 during the rotation of therotatable shafts 2A. As shown in FIG. 1(B), the agitating fins 2Bprovided on the two rotatable shafts partly overlap each other whenviewed from the top. The overlapped area of the agitating fins 2Bincreases the agitating capability.

The refiner 1 is provided with an inlet 3 at lower part thereof tosupply a crude cyclic ester. The inlet 3 is furnished with a means forsupplying the crude cyclic ester to the inside of the refiner 1, such asa screw conveyer (not shown).

The refiner 1 is also provided at an upper part thereof with an outlet 4to take out the refined ester crystal and a means 5 for supplying amelted liquid for the refined crystal component from the outside of therefiner 1. The means 5 is a conduit for supplying a predetermined amountof the melted liquid having an accurately known temperature.

The refiner 1 is provided with a filter 6 at a lower part thereof and animpurity outlet 7 at the bottom thereof.

The other type of refiner in FIG. 2 has the same reference numbers asthose of FIG. 1.

A upright type refiner 1 in FIG. 2 is provided at a lower part thereofwith an inlet 3 having a screw conveyer 3A therein to supply a crudecyclic ester including impurities and, at an upper part thereof, amelting heater 14 to melt the ester crystal. The melted liquid melted bythe heater 14 are brought into counterflow-contact with the cyclic estercoming upwardly in the refiner 1 to refine the crystal. The refinedcrystal is taken out from an outlet 4 provided at an upper part of therefiner 1. An agitator 2 of the refiner 1 is provided with a rotatableshafts 2A having agitating fins 2B, unraveling and making the crystalsupplied from the inlet 3 flow upwardly.

How to refine the two types of crude cyclic esters using either of therefiners in FIGS. 1 and 2 will be described.

(1) The Cyclic Ester Having No Solvent

The cyclic ester crystal obtained by depolymerizing a hydroxycarboxylicoligomer is successively supplied to the refiner 1 from the inlet 3 bythe supplying means such as the screw conveyer. The supplied crystal isagitated by the agitating fins 2B and flows upwardly inside the refiner1. A part of the crystal turns to a downflowing melted liquid togetherwith the melted cyclic ester, which is melted by the melting heater 14or supplied from the outside by the melted liquid supplying means 5. Thedownflowing melted liquid is brought into counterflow-contact with theupflowing crude cyclic ester crystal to clean off the impurities adheredto the surface of the crystal and sweat out the impurities caught insidethe crystal, thus refining the crude cyclic ester. The refined crystalis taken out from the outlet 4, and the removed impurities are exhaustedfrom the impurity outlet 7 through the filter 6.

(2) The Cyclic Ester Having High-Boiling Point Organic Solvent AdheredThereto

The cyclic ester crystal having a high-boiling point organic solventadhered thereto is obtained by depolymerizing and distilling ahydroxycarboxylic oligomer with the high-boiling point organic solvent,cooling a distilled product at a temperature lower than the meltingpoint of the cyclic ester, and separating the thus produced product,which comprises the cyclic ester crystal and a slurry of thehigh-boiling point organic solvent, into a solid and a liquid. The thusobtained cyclic ester crystal is successively supplied from the inlet 3by the supplying means such as the screw conveyer. It is preferable thatthe content of the cyclic ester crystal in the supplied substances ismore than 50%. The supplied crystal is agitated by the agitating fins 2Band flows upwardly inside the refiner 1. A part of the crystal turns toa downflowing melted liquid together with the melted cyclic ester, whichis melted by the melting heater 14 or supplied from the outside by themelted liquid supplying means 5. The downflowing melted liquid isbrought into counterflow-contact with the upflowing crude cyclic estercrystal to clean off the impurities adhered to the surface of thecrystal and sweat out the impurities caught inside the crystal, thusrefining the crude cyclic ester. The refined crystal is taken out fromthe outlet 4 and removed impurities are exhausted from the impurityoutlet 7 through the filter 6.

EXAMPLE

As an example, 18.3 kg/hour of a glycolide having a purity rate of99.99% is obtained by supplying 22.1 kg/hour of a mixture comprising91.1 mass % of a glycolide and 8.9 mass % of dibutyl phtalate having aboiling point of 340° C. to the refiner 1 shown in FIG. 2, which has twocylinders each having an internal diameter of 75 mm and a height of 2000mm.

Industrial Application

The present invention comprises the steps of supplying the crud cyclicester to a vertically extending cylindrical refiner from an inletprovided at a lower part of the refiner, agitating the crude cyclicester by an agitator provided in the refiner to make the crude cyclicester flow upwardly, refining the cystal of the crude cyclic ester bycounterflow-contact between the upflowing crude cyclic ester and adownflowing melted liquid which contains a refined crystal component,and taking out the refined crystal of the crude cyclic ester from anoutlet provided at upper part of the refiner. Consequently, the presentinvention has the following advantages.

(1) No additional solvent or cleaning liquid for the refining process isrequired, unlike the conventional refining method.

(2) Since no additional solvent or cleaning liquid is used, the refiningequipment is simplified. That is, various processes including cleaningof the high-boiling point organic solvent, evaporation and separation ofthe high-boiling point organic solvent and the cleaning liquid, dryingof removing the cleaning liquid adhered to the refined crystal, coolingand collecting of the cleaning liquid discharged from the dryingequipment are not required. Accordingly, many machines and ancillaryfacilities such as a pump, storage tank, and pipe are not necessary. Inaddition, electricity, steam, and cooling water are saved.

(3) Even if impurities are caught inside the crystal, and a plurality oftimes of re-crystallization are required under the conventional method,highly pure crystal is produced by the invention.

(4) Since the steps of the whole process are successively operated, theoperation control is easy.

(5) The manufacturing cost is reduced.

What is claimed is:
 1. A method of refining a crystal of a crude cyclicester, comprising the steps of: supplying said crude cyclic ester to avertically extending cylindrical refiner from an inlet provided at alower part of said refiner; agitating said crude cyclic ester by anagitator provided in said refiner to make said crude cyclic ester flowupwardly; refining said crystal of said crude cyclic ester bycounterflow-contact between said upflowing crude cyclic ester and adownflowing melted liquid which contains a refined crystal component;and taking out said refined crystal of said crude cyclic ester from anoutlet provided at an upper part of said refiner.
 2. The methodaccording to claim 1, wherein said melted liquid is supplied fromoutside of said refiner.
 3. The method according to claim 1, wherein atleast one high-boiling point organic solvent having a boiling pointgreater than a melting point of said crystal of said crude cyclic esteris adhered to said crystal of said crude cyclic ester.
 4. The methodaccording to claim 2, wherein at least one high-boiling point organicsolvent having a boiling point greater than a melting point of saidcrystal of said crude cyclic ester is adhered to said crystal of saidcrude cyclic ester.
 5. The method according to claim 1, wherein saidcyclic ester is a hydroxycarboxylic acid cyclic ester.
 6. The methodaccording to claim 2, wherein said cyclic ester is a hydroxycarboxylicacid cyclic ester.
 7. The method according to claim 3, wherein saidcyclic ester is a hydroxycarboxylic acid cyclic ester.
 8. The methodaccording to claim 4, wherein said cyclic ester is a hydroxycarboxylicacid cyclic ester.
 9. The method of according to claim 5, wherein saidhydroxycarboxylic acid cyclic ester is a glycolic acid dimeric cyclicester.
 10. The method of according to claim 6, wherein saidhydroxycarboxylic acid cyclic ester is a glycolic acid dimeric cyclicester.
 11. The method of according to claim 7, wherein saidhydroxycarboxylic acid cyclic ester is a glycolic acid dimeric cyclicester.
 12. The method of according to claim 8, wherein saidhydroxycarboxylic acid cyclic ester is a glycolic acid dimeric cyclicester.